1. Technical Field of the Invention
The present invention relates to an azimuth detecting apparatus that detects the azimuth of a target by sending and receiving continuous waves, and to a radar apparatus that employs the azimuth detecting apparatus.
2. Description of the Related Art
Conventionally, a radar apparatus is installed in a motor vehicle to detect the presence of a target (for example, a roadside object such as a guardrail or a preceding vehicle) by sending and receiving continuous waves (i.e., radar waves).
More specifically, to detect the azimuth of the target and thereby determine the position of the target, the radar apparatus employs an array antenna as a receiving antenna. The array antenna is configured by, for example as disclosed in Japanese Patent First Publication No. H8-181537, arranging a plurality of antenna elements at predetermined equal intervals.
Since the radar apparatus is installed in the vehicle that usually runs at high speed, it is required for the radar apparatus to be capable of detecting the azimuth of a target present at a distant location (for example, away from the vehicle by 100 to 200 m). Further, to meet this requirement, the antenna elements are generally so arranged as to narrow the beamwidth of the array antenna (i.e., sharpen the main lobe of the array antenna).
Consequently, the angular range of an azimuth detection area of the radar apparatus becomes narrow. Here, the azimuth detection area is defined such that the radar apparatus can detect the azimuth of a target only when the target is located in the area. In addition, hereinafter, the narrow angular range is to be referred to as conventional angular range, and the azimuth detection area with the narrow angular range is to be referred to as conventional azimuth detection area. As a result, it is difficult for the radar apparatus to detect, in an early stage, another vehicle that cuts in front of the vehicle from the side.
Accordingly, it is desired for the radar apparatus to have a wider angular range of the azimuth detection area than the conventional angular range while keeping the capability of detecting the azimuth of a distant target.
To meet the above desire, one may consider configuring the receiving antenna with both a first array antenna having a plurality of antenna elements equally spaced at first intervals and a second array antenna having a plurality of antenna elements equally spaced at second intervals that are narrower than the first intervals.
However, with the above configuration, the receiving antenna would be enlarged, resulting in an increase in the manufacturing cost. On the other hand, the space in the vehicle available for installation of the radar apparatus is generally limited, and it is thus undesirable to enlarge the receiving antenna.